The absorptive power of a body (or a surface) is defined as the ratio of the energy absorbed in a given time (or in a certain time) to the radiant energy incident on it at the same instant of time.
Therefore absorptive power, a = Amount of energy absorbed / Amount of energy incident
The absorptive power of a black body is 1 because it absorbs the radiant energy of all wavelength incidents on it. A good absorber is a good radiator. So, the body which has higher absorptive power has the higher emissive power also.